The present invention relates to a noise suppressor for reducing noise contained in transmitted/received speech signal, which is used in radio communication apparatuses of various digital communication methods, including a digital mobile phone system.
A telephone service using speech communication is known as a basic service of mobile communication. A mobile telephone system first began with an analog method, but now a digital method is prevailing.
In the digital method, an A/D converter is needed to convert analog speech signals to digital signals. However, simple A/D conversion requires a coding rate of about 100 kbps. Considering limited radio wave resources, it is necessary to compress the digital signals to 1/10 to  1/20. To meet the demand, a high-efficiency speech coding method, generally called speech compression, is employed and it is embodied as a speech CODEC. 
In current mobile communications, a speech CODEC with a coding rate of about 3.5 kbps to 32 kbps is used. In the low-rate CODEC, the coding rate is decreased by utilizing the characteristics of speech signals as much as possible. As a result, even if an adequate quality of speech is obtained, the reproducibility and quality of “sound” other than speech tend to deteriorate.
A low-rate speech CODEC is used as an application in mobile phones which are often used outdoors. In some cases, mobile phones are used in an environment with large background noise.
If background noise is input to the low-rate speech CODEC which is designed mainly for “speech”, the speech quality will vary. The clearness and quality of speech will deteriorate in the environment with background noise.
As techniques for solving this problem, attention has recently been paid to noise suppressors (or noise cancelers) which are designed to suppress background noise taken in through microphones and to deliver only speech to the speech CODEC.
For example, a noise canceler is described in the chapter “Half-Rate Speech CODEC” in the “Personal Digital Cellular Telecommunication System RCR STD-27” published by Association of Radio Industries and Businesses (ARIB) in Japan.
New speech CODECs have been developed by technical innovations. There is a recent trend of multi-mode, in other words multi-algorithm, wherein new CODECs are introduced in systems to achieve two-algorithm switching (two speech CODECs can be switched) or three-algorithm switching (three speech CODECs can be switched).
On the other hand, like EVRC (Enhanced Variable Rate Codec) known as the TIA (Telecommunications Industry Association) standard IS-127 in the U.S.A. or AMR (Adaptive Multi Rate), multi-rate systems have been proposed wherein one CODEC is used while plural different coding rates are supported. Moreover, a hands-free function that enables a user to make calls without having to lift his/her handset has been provided for the user's convenience.
However, in the conventional multi-mode or multi-rate communications apparatus, the noise suppressor may not fully function due to mismatching between the speech CODEC and noise suppressor in a certain selected mode or rate. As a result, high-quality transmitted speech or received speech cannot be obtained.
Furthermore, in the conventional communications apparatus with the hands-free function, in accordance with switching between the hands-free algorithm and non-hands-free algorithm, a speech input path to the noise suppressor may vary via a microphone, an analog amplifier, etc. or speech input characteristics may vary. Besides, if the environment of use changes, for example, when a new device such as an echo canceler is provided in the signal path for echo control, the noise suppressor cannot fully function and high-quality transmitted speech or received speech cannot be obtained.